C1 — Lipids Flashcards
Glycerol’s structure + solubility
Structure: Molecular formula C3H8O3, an alcohol, each C bears a hydroxyl (-OH) group
Solubility: Soluble in water because of its polar –OH group. hydrophilic (when alone. When in triglycerides, due to the lack of free -OH groups, there is thus no polar region, hence triglycerides are hydrophobic due to the hydrophobic fatty acid chains)
Classification of lipids
Simple lipids: consist of an alcohol (usually glycerol) linked to one or more fatty acids via an ester linkage
(EG triglycerides)
Compound lipids: Compound lipids are esters of fatty acid(s) and an alcohol plus other chemical groups, such as
phosphate and sugar
(EG phospholipids, glycolipids)
Lipids derivatives: Cholesterol possesses a carbon skeleton made up of three fused six-membered and one five-membered ring
(EG steroid cholesterol)
Fats (triglycerides) + properties
Consists of a glycerol molecule linked to 3 fatty acids by ester linkages.
Properties:
- The melting point of fats increases with hydrocarbon chain length. The longer the hydrocarbon chains, the more extensive the hydrophobic interactions between the chains, the higher the melting points (as more thermal energy is required to break the bonds). Hydrophobic interaction is a weak bond that exists between hydrophobic molecules. It is a force of attraction between non-polar molecules.
- The melting point of fats decreases as the degree of unsaturation of fatty acids tails increases. The kinks where the double bonds are located prevent the molecules from packing closely. When the fatty acids tails are less closely packed, hydrophobic interactions are less extensive. Hence, less thermal energy is required to break enough of these interactions to liquefy the triglycerides.
(Saturated fats -> solid at room tempt)
Fats structure + functions
More efficient energy stores than carbs:
Triglycerides have a higher proportion of C and H atoms compared to O atoms -> have a greater number of carbon atoms per unit mass than carbohydrates. Upon oxidation, triglycerides release a larger amount of energy, about more than twice as much as a carb..
More metabolic water released when oxidised as compared to carbs:
Highly reduced molecules as they have two-fold more hydrogen atoms per unit mass than carbohydrates.
Hydrophobic as C-H bonds are non-polar -> no associated water molecules stored along with triglycerides, thus they have no extra weight due to water of hydration.
Triglycerides do not affect water potential of cells when stored in large amounts.
The absence of water of hydration in triglycerides allows for an animal’s body mass to be kept to a minimum to facilitate locomotion.
Triglycerides are a good thermal insulator and hence acts as a layer of fat beneath the skin that insulates the body.
Hydrocarbon tails are non-polar -> weak hydrophobic interaction occur between triglyceride molecules -> slide under pressure -> adipose tissue around the vital organs helps to cushion and protect vital organs against physical impacts
Lower molecular weight than water per unit volume. Being less dense than water, fats aid buoyancy of aquatic animals
Phospholipids (compound lipids)
Consist of one glycerol and two fatty acid chains. The third OH group of glycerol is joined to a negatively-charged phosphate group by a phosphoester linkage. Additional small molecules, usually charged or polar (e.g. serine, choline and inositol) can be linked to the phosphate group to form a variety of phospholipids. The two fatty acids are linked to the glycerol by an ester linkage.
Properties:
Amphipathic behaviours towards water
-> 2 fatty acids that comprise the hydrocarbon tails are non-polar and hence hydrophobic
-> The phosphate group and its attachments form a polar / charged hydrophilic head that has an affinity for water.
Examples: cell membranes of plants, bacteria and animals
Phospholipids structure + functions
Phospholipids are amphipathic molecules each with 2 non-polar, hydrophobic fatty acids ‘tails’ and a charged, hydrophilic phosphate ‘head’.
- Phospholipids form a selectively permeable cell membrane where hydrophilic heads are exposed to aqueous medium while the hydrophobic tails are in contact with those of neighbouring molecules but excluded from aqueous medium in the non-polar interior of the bilayer. This forms an effective barrier / boundary between cell and its external environment
- Phospholipids form a liposome / vesicle when a lipid bilayer folds back on itself to form a hollow sphere. Liposomes are used as vesicles for storage and transport of cellular products (e.g. proteins, lipids), as well as for digestion of waste (in vesicles known as lysosomes) and for drug delivery in humans. By forming vesicles, bilayer sheets avoid exposing their hydrophobic edge regions, achieving maximal stability in their aqueous environment.
- Phospholipids form micelles used for the transport of fats between the gut and body tissues.
- Phospholipids also form bilayers
Hydrophobic interactions exist between fatty acids tails
- Integrity of the membrane bilayer/liposomes/vesicles/micelles is maintained due to large number of interactions
- individual hydrophobic interactions are weak, permitting lateral movement of phospholipids, which account for membrane fluidity
Most phospholipids contain choline -> choline is important for the synthesis of acetylcholine, a neurotransmitter.
Hydrophobic fatty acid chains, unsaturated fatty acid chains and saturated fatty acid chains affect the role of these molecules in the cell surface membrane.
Glycolipids
Composed of two hydrophobic hydrocarbon tails, and a polar, short carbohydrate chain (less than 15 sugar residues) with no phosphate that is joined to the glycerol’s -OH group by a glycosidic bond.
Examples: Found in nerve cells, the brain and red blood cells, involved in cell recognition and signalling. Found at the cell surface membrane facing the exterior environment.
Glycolipids structure + functions
A carbohydrate chain that is attached to the glycerol…
- Is found at the cell surface membrane facing the exterior environment.
- Serves as a marker that distinguishes one cell from another in cell-cell recognition. Cells recognize other cells by binding to these carbohydrate chains.
- Involved in cell-cell adhesion in tissue formation.
Hydrophobic interactions exist between fatty acids tails serve to anchor the entire glycolipids at the cell surface membrane.
Describe what is meant by a condensation reaction. [2]
- Formation of an ester bond between one glycerol and one fatty acid chain
- Resulting in loss of one water molecule
Explain how the structure would be different for an unsaturated triglyceride, as compared to a saturated triglyceride. [2]
- Presence of one or more carbon-carbon double bond within the fatty acid chain
- Presence of kink in the fatty acid chain whenever a double bond occurs
The eukaryotic cell surface membrane contains phospholipids, cholesterol and proteins. Describe how a phospholipid molecule differs from a triglyceride molecule. [2]
- Ref to 2 fatty acid chains for phospholipids but 3 fatty acid chains for triglycerides
- Ref to presence of phosphate head for phospholipids but absence of phosphate head for triglyceride
Describe the arrangement of phospholipids in cell membranes. [2]
And Explain how the structure of phospholipids is related to this arrangement in cell membranes. [3]
- Ref. to phospholipids being arranged in a double layer / bilayer ;
- Ref. to phosphate head of phospholipid facing outwards into the aqueous environment, hydrocarbon tails of phospholipid facing inwards / away from the aqueous environment ;
- Ref. to amphipathic nature of phospholipid, which has a hydrophilic head and hydrophobic tails ;
- The hydrocarbon / fatty acid tails are held together by hydrophobic interactions forming a hydrophobic core ;
- The negatively-charged phosphate head interact with the aqueous environment both inside and outside of the cell ;
Identify the main structural feature common to all 3 molecules (phospholipids, glycolipids, triglycerides). [1]
All 3 molecules contain 2 or more hydrocarbon/fatty acid chains.
Structure [2] and role in cells [2] difference between phospholipid and Glycolipid.
Structure:
Phospholipid has 2 hydrophobic fatty acids chains and one hydrophilic phosphate group but Glycolipid contains a sugar/carbohydrate oval entry bound to 2 hydrocarbon tails.
Role in cells:
Any one (P):
- Phospholipid form a lipid bilayer which allows for compartmentalization within cell
- phospholipid form a lipid bilayer which act as a barrier to control movement of molecules and ions
- Phospholipid can form vesicles for transport of cellular substances within the cell
any one (G):
Glycolipid, present on the external face of the plasma membrane, is required
- For cell-to-cell recognition
- for cell-to-cell adhesion
- to form cell receptors
Explain the differences in solubility between triglycerides and the products of their hydrolysis. [3] (3 points -> structure, solubility, fatty acid chains)
Triglycerides are insoluble in aqueous medium as they do not have any hydrophilic groups [1]
Fatty acid is insoluble in aqueous medium as the long hydrocarbon/fatty acid chains that form the bulk of the molecule are hydrophobic [1]
Glycerol is soluble in aqueous medium as it contains 3 hydroxyl groups which are hydrophilic [1]
Explain how the presence of double bonds affects membrane integrity at low temperatures. [3] (membrane integrity -> membrane fluidity)
- Double bonds cause kinks in fatty acid tails to prevent close packing of phospholipids (REJECT kinks in membrane)
- hydrophobic interactions are less extensive, and less thermal energy is required to break enough of these interactions (especially need to mention thermal energy when question asks for mp and bp)
- hence maintaining membrane integrity at low temperature by keeping the membrane fluid, increasing membrane fluidity.
